The present invention relates to a slew-ring bearing for application to heavy equipment such as large excavators.
Large hydraulic excavators used for digging rock such as in coal beds utilize a large bearing called a slew ring bearing for rotatably coupling a large superstructure to a base frame seated on the ground. A shovel or bucket is pivotally attached to the distal end of an articulating boom that is pivotally fastened to the superstructure. Pinion gears fixed to the superstructure engage gear teeth on an outer ring of the slew ring. The outer ring is affixed to the base frame and coupled to an inner ring attached to the superstructure by rows of cylindrical rolling elements (rollers). The conventional slew ring used has 3 rows of rollers; upper and lower ones whose axes of rotation are horizontal and an intermediate one whose axis of rotation is vertical. Downward (vertical) or thrust loads are resisted by rollers in the upper row. Radial or horizontal loads are taken by the intermediate row of rollers while the lower row of rollers, termed the retaining or lift row, prevents tipping by resisting the vertically upward thrust caused thereby.
It was found in a particular application of such bearings in Northern locations where applied movement loads (force x distance) caused by excavator bucket operation reached 15 million foot pounds, that conventional slew ring bearings failed after an average of only 6,100 hours of operation. Some failures occurred after only 800 hours. Acceptable lifetimes between failure are in the range of 20,000 to 30,000 hours. A detailed two year study into the causes of such premature failure pointed to several contributing factors. A first area of concern was in the configuration of the rollers. Since the applied loads are generally either vertical or horizontal, horizontal loads are taken by the intermediate row of rollers, downward loads by the upper row of rollers and upward thrusts caused by tipping movements applied to the inner ring are taken by the lower set of rollers. The study determined that there were two problems with the design having three rows of rollers. First, the entire downward vertical thrust is taken by only one row of rollers. SeCond, the requirement of fitting three rows of rollers into grooves in the inner ring limits the maximum size of the rollers and decreases reliability due to having a greater number of components as well as relatively small rollers.
Another problem had to do with the tendency of the rollers themselves to become damaged and the precision ground raceways to be scratched, scored or galled as a result. Furthermore, on the existing design there is no positive way of predicting or anticipating catastrophic failure in the form of cessation of rotation and scheduling maintenance without shutting down the machine entirely together with other activities dependent on machine operation.
Finally, distortion of the inner ring was thought to be an important contributing factor of failure causing uneven load distribution with large and disproportionate loads on selected rollers being a result.
Unfortunately, replacement or repair of the slew ring in an excavation takes several days as the entire assembly must be disassembled to change the bearing.
Accordingly, it is an object of the present invention to provide an improved slew ring bearing design. More particularly, it is an object to provide a slew ring bearing design with improved reliability and longer operating life.
It is a further object of the invention to provide a slew ring bearing design which can better withstand distortion and changing loads.
It is yet a further object of the invention to provide a slew ring bearing with a means for easily inspecting, removing and replacing rollers and spacers if necessary.